Bee pollen is the pollen ball that has been packed by worker honeybees into pellets. Bee bread is the bee pollen with added honey and bee secretions and stored in brood cells, chambers of wood and mud created by female ground-nesting bees. When the pollen ball is complete, a single female lays an egg on top of the pollen ball, and seals the brood cell. Pollen balls are harvested as food for humans. Bee pollen is sometimes referred to as ambrosia. Foraging bees bring pollen back to the hive, where they pass it off to other worker bees, who pack the pollen into cells with their heads. During collection and possibly packing, the pollen is mixed with nectar and bee salivary secretions. Bee pollen is the primary source of protein for the hive.